Times have changed considerably since then. Most modern hard disk drives are much more efficient at reading, writing, and caching. Furthermore, modern server operating systems, especially Linux distributions and BSD variants, use much more sophisticated file systems that make better use of the available space. Therefore, in most situations, defragmenting is not necessary.
In unscientific terms, Fragmentation occurs when a chunk of data is written to the hard drive, then deleted, and then another chunk is written in another space, rather than in the empty space that was left from the previous chunk. Pretty soon, you start to have a lot of these empty spaces where chunks used to be.
Efficient file systems like Ext3 and Ext4 do a better job of writing to disks in an orderly fashion, keeping the empty spaces to a minimum. Linux users, therefore, generally do not experience any performance decrease, even when their drives become fragmented.
In some rare instances, it may be possible for Linux file systems to become fragmented after years of heavy writing and deleting of data. On a server, however, this is typically unlikely. Furthermore, in order to even check the drive for fragmentation, you would have to unmount the filesystem, which would invariably result in down time. The disadvantages simply outweigh the benefits.